The present invention relates to a multiple-directional switching valve which has a bucket-leveling function in which a bucket is maintained at a level position by supplying a return pressured oil drained from a boom cylinder to a bucket cylinder when a boom is operated by supplying the pressured oil to the boom cylinder.
Conventionally, a multiple-directional switching valve which is used for a construction vehicle and the like and has a bucket-leveling function in which a bucket is maintained at a level position by supplying a return pressured oil drained from a boom cylinder to a bucket cylinder when a boom is operated by supplying the pressured oil to the boom cylinder. See, for example, Japanese Patent Laid-Open Publication No. 7-252857 (page 5, FIGS. 4 and 5), and Japanese Patent Laid-Open Publication No. 10-219730 (pages 4 and 5, FIGS. 1 through 3) (its corresponding U.S. Pat. No. 5,797,310). A hydraulic circuit disclosed in the former patent publication shows a multiple-directional switching valve having such bucket-leveling function in which the bucket is maintained in a position with a specified angle with respect to a ground surface by supplying a pressured oil drained from a second chamber (rod-side chamber) of a second cylinder (boom cylinder) to a first chamber (head-side chamber) of a first cylinder (bucket cylinder) when a lift arm (boom) is raised and by supplying a pressured oil drained from a first chamber of the second cylinder to a second chamber of the first cylinder when the lift arm is lowered. Further, an automatic-leveling control hydraulic device disclosed in the latter patent publication shows a multiple-directional switching valve having such bucket-leveling function as well.
Herein, the above-described hydraulic circuit of the former patent publication does not have a canceling function to cancel the bucket-leveling function properly. Accordingly, in the event that the boom is lowered and the bucket is dumped during, for example, a boring, there was a concern that a dumping speed of the bucket may be delayed compared with an operation expected by an operator or a dumping operation may not be performed properly because a bucket-leveling mechanism moves the bucket toward curing direction. Namely, the above-described hydraulic circuit of the former patent publication had a problem that its operation may deteriorate when both the boom and the bucket are operated at the same time.
Meanwhile, the above-described latter patent publication discloses an example of a means for canceling the bucket-leveling function in which a check valve is changed to a solenoid valve in which the solenoid valve is operated to allow a return oil from the boom cylinder to flow into a tank via the solenoid valve when the bucket-leveling function is required. However, there was a concern that since the return oil drained from the boom cylinder flows into the tank via a valve, a flow passage may be narrowed by the valve and there may occur some pressure here, so that the oil may flow into the bucket cylinder. As a result, there was a problem that such cancellation of the bucket-leveling function could not be provided properly.
Also, in the above-described hydraulic circuit of the former patent publication, a directional switching valve for boom, a flow divider system and a directional switching valve for bucket are located on an identical plane. Accordingly, there was a problem that the multiple-directional switching valve may require a relatively long shape in its longitudinal direction and thereby it may be difficult to provide an enough space for its disposition.
Herein, there are also other prior art disclosing similar multiple-directional switching valve, such as Japanese Patent Laid-Open Publication No. 2-96028, U.S. Pat. Nos. 4,408,518 and 5,447,094.